1. Field of the Invention
The present invention relates to the production integrated circuits. More particularly, the invention relates to nanoporous dielectric coatings useful in the production of integrated circuits.
2. Description of Prior Art
It is known in the art that, in the production of integrated circuits, the problems of interconnect RC delay, power consumption and crosstalk become more significant as feature sizes approach 0.25 .mu.m and below. It has been found that the use of low dielectric constant (K) materials for interlevel dielectric and intermetal dielectric applications partially mitigate these problems. However, each of the material candidates which are under consideration by the industry, having dielectric constants significantly lower than the currently employed dense silica, suffer from disadvantages. Most low dielectric constant materials developments use spin-on-glasses and fluorinated plasma chemical vapor deposition of SiO.sub.2 with K of &gt;3. Some organic and inorganic polymers have dielectric constants in the range of about 2.2 to 3.5, however, these polymers exhibit problems of low thermal stability, and poor mechanical properties including low glass transition temperature, and sample outgassing, thereby raising questions concerning their long term reliability questions.
Density, or its inverse, porosity, is the key parameter controlling property of importance for dielectrics. Higher porosity materials not only lead to a lower dielectric constant than dense materials, but they also allow additional components and processing steps to be introduced. As density decreases, dielectric constant and mechanical strength decrease, however the pore size increases. Important issues relating to porous materials include pore size; the strength decrease associated with porosity; and the role of surface chemistry on dielectric constant, loss and environmental stability.
One solution to these issues is the use of nanoporous silica, which can have dielectric constants in the range of about 1 to 3. Nanoporous silica is particularly attractive due to the ability to carefully control its pore size and pore distribution, and because it employs similar precursors such as tetraethoxysilane (TEOS), as is presently used for spin-on glass (SOG's), and CVD SiO.sub.2. In addition to having low dielectric constants, nanoporous silica offers other advantages for microelectronics, including thermal stability up to 900.degree. C.; small pore size (&lt;&lt;microelectronics features); use of materials, namely silica and its precursors, that are widely used in the semiconductor industry; the ability to tune dielectric constant over a wide range; and deposition using similar tools as employed for conventional spin-on glass processing. EP patent application EP 0 775 669 A2, which is incorporated herein by reference, shows a method for producing a nanoporous silica film with uniform density throughout the film thickness.
Nanoporous silica films are typically fabricated by methods such as dip-coating or spin-coating. When spin-coating, a mixture of a solvent and a silica precursor is deposited on a substrate wafer which is placed on a chuck in an open cup. The substrate is spun at several thousand rotations per minute (rpm's) in order to achieve a uniformly thin film on the substrate. Typically, the substrate is open to the atmosphere such that excess fluid can be flung from the substrate edge. However, turbulence around the substrate often results in a film which is not completely uniform, and which may vary in thickness. Turbulence is believed to cause defects such as striations, which are thickness gradients in the deposited film that are started at the center of the substrate and spiral radially outward to the edge of the substrate. This can cause a film to be nonuniform.
The present invention offers a solution to these problems. It has been unexpectedly found that using a closed cup when spin-coating will reduce turbulence around the substrate and result in a more uniform film. According to the present invention, a cover is placed over the substrate wafer so that the cup, cover, and substrate spin simultaneously. This simultaneous spinning eliminates turbulence that is normally found in traditional spin coating processes where only the substrate spins and the cup is stationary. Subsequently, vapors of water and a base such as ammonia are injected into the cover of the cup. Because of the lower turbulence due to the covering of the cup, the silica precursor is uniformly exposed to the vapors and is polymerized until it forms a gel. After this exposure, the substrate is ready for curing. Using this approach, a nanoporous silica film is obtained with uniform density and film thickness. In another embodiment of the invention, the precursor can be reacted with the base and water vapor after removal from the cup.